Supervolcano Not to Blame for Humanity's Near-Extinction

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A supervolcanic eruption thought to have nearly driven humanity
extinct may not have endangered the species after all, a new
investigation suggests.

Supervolcanoes are capable of eruptions dwarfing anything
ever seen in recorded history, expelling thousands of times more
magma and ash than even a Mount St. Helens or Pinatubo. A
supervolcanic eruption could wreak as much havoc as the impact of
a mile-wide asteroid,by blotting out the sun with ash, reflecting
its rays and cooling the Earth — a phenomenon called a "volcanic
winter." Adozen or so supervolcanoes exist today,
some of them lying
at the bottom of the sea.

The largest supervolcano eruption of the past 2.5 million years
was
a series of explosions of Mount Toba on the Indonesian island
of Sumatra about 75,000 years ago. Researchers say Toba spewed
out a staggering 700 cubic miles (2,800 cubic kilometers) of
magma, equivalent in mass to more than 19 million Empire State
Buildings. By comparison, the infamous blast from the volcanic
Indonesian island of Krakatoa in 1883, one of the
largest eruptions in recorded history, released about 3 cubic
miles (12 cubic km) of magma.

About the same time the eruption took place, the number of modern
humans apparently dropped cataclysmically, as shown by genetic
research. People today evolved from the few thousand survivors of
whatever befell humans in Africa at the time. The giant plume of
ash from Toba stretched from the South China Sea to the Arabian
Sea, and in the past investigators proposed the resulting
volcanic winter might have caused this die-off. [ Countdown:
History's Most Destructive Volcanoes ]

However, recently scientists have suggested that Toba did not
sway the course of human history as much as previously thought.
For instance,
prehistoric artifacts discovered in India and dating from
after the eruption hinted that people coped fairly well with any
effects of the eruption.

Now researchers have found that the evidence shows Toba didn't
actually cause a volcanic winter in East Africa where humans
dwelled.

"We have been able to show that the largest volcanic eruption of
the last two million years did not significantly alter the
climate of East Africa," said researcher Christine Lane, a
geologist at the University of Oxford.

Lane and her colleagues examined ash from Toba recovered from mud
extracted from two sites at the bottom of Lake Malawi, the second
largest lake in the East African Rift Valley.

"We first started looking for the Toba ash a few years back, but
it's a bit like looking for a needle in a haystack, so it took a
while," Lane told OurAmazingPlanet. "Between myself and my
co-author Ben Chorn, we systematically processed every centimeter
of sediment between 24 to 46 meters [78 to 150 feet] depth in the
central basin core. The layer is so small that if we leave any
gaps in our search, we could miss it completely."

Their analysis discovered that a thin layer of ash in this
sediment about 90 feet (27 m) below the lake floor was from the
last of the Toba eruptions, known as Youngest Toba Tuff.

"The Toba
super-eruption dispersed huge volumes of ash across much of
the Indian Ocean, Indian Peninsula and South China Sea," Lane
said. "We have discovered the layer of volcanic ash was carried
about twice the distance as previously thought, over more than
7,000 kilometers [4,350 miles]."

The amount of ash found in the Malawi sediment core (a
cylindrical log of sediment drilled from the ground), was more
than the scientists expected to find.

"I was surprised to find so much ash in the Lake Malawi record,"
Lane added. "The ash is very tiny, composed of shards of volcanic
glass smaller than the diameter of a human hair. Nevertheless, in
a lot of records I have worked on previously, even within just a
few hundreds of miles of an eruption center, we sometimes only
find less than 100 shards of glass within a gram of sediment. In
Malawi, we have thousands of shards of glass per gram, which
really shows how voluminous the Youngest Toba Tuff was."

Quick recovery

If the area had seen dramatic cooling because of all the ash
spewed into the atmosphere, living matter near the lake surface
would likely have died off, significantly altering the
composition of the lake's mud. However, when the researchers
investigated algae and other organic matter from the layer that
contained the ash from Toba, they saw no evidence of a
significant temperature drop in East Africa. Apparently, "the
environment very quickly recovered from any atmospheric
disturbance that may have occurred," Lane said.

"It is important to realize that every volcanic eruption is
different and the Youngest Toba Tuff provides only one example,"
Lane said. "The impact of an eruption depends not just on the
amount of ash erupted, but also the composition and volume of
aerosols, how high in the atmosphere the ash is injected and the
meteorological conditions at the time."

As for what might explain the near-extinction humanity apparently
once experienced, perhaps another kind of catastrophe, such as
disease, hit the species. It may also be possible that such a
disaster never happened in the first place — genetic research
suggests modern humans descend from a single population of a few
thousand survivors of a calamity, but another possible
explanation is that modern humans descend from a few groups that
left Africa at different times.

Future research will analyze what effects Toba may or may not
have had on other lakes in East Africa.

"Whilst from this we can hypothesize that the global climatic
impact was not as dramatic as some have suggested, we will need
to find similarly high-resolution records of past climate from
other regions that also contain Youngest Toba Tuff in order to
definitively test this," Lane said.